Advanced synthesis of materials for intermediate-temperature solid oxide fuel cells

Abstract

Solid-oxide fuel cells (SOFCs) technology has a substantial potential in the application of clean and efficient electric power generation. However, the widespread utilization of SOFCs has not been realized because the cost associated with cell fabrication, materials and maintenance is still too high. To increase its competitiveness, lowering the operation temperature to the intermediate range of around 500–800 °C is one of the main goals in current SOFCs research. A major challenge is the development of cell materials with acceptably low ohmic and polarization losses to maintain sufficiently high electrochemical activity at reduced temperatures. During the past few decades, tremendous progress has been made in the development of cell materials and stack design, which have been recently reviewed. SOFCs are fabricated from ceramic or cermet powders. The performances of SOFCs are also closely related to the ways in which the cell materials are processed. Therefore, the optimization of synthetic processes for such materials is of great importance. The conventional solid-phase reaction method of synthesizing SOFCs materials requires high calcination and sintering temperatures, which worsen their microstructure, consequently, their electrochemical properties. Various wet chemical routes have recently been developed to synthesize submicro- to nano-sized oxide powders. This paper provides a comprehensive review on the advanced synthesis of materials for intermediate-temperature SOFCs and their impact on fuel cell performance. Combustion, co-precipitation, hydrothermal, sol–gel and polymeric-complexing processes are thoroughly reviewed. In addition, the parameters relevant to each synthesis process are compared and discussed. The effect of different processes on the electrochemical performance of the materials is evaluated and optimization of the synthesis processes is discussed and some emerging synthetic techniques are also briefly presented.

Introduction

The need for clean and efficient energy conversion is a critical technological and economic challenge. Fuel cells are a promising way of generating electric power from a variety of fuels and have a low environmental impact and high efficiency [1], [2], which may play important role in future clean energy generation. Among the various kinds of fuel cells, solid-oxide fuel cells (SOFCs) are advantageous because they have the highest energy conversion efficiency and excellent fuel flexibility, owing to their high operation temperature (typically 500–1000 °C). Furthermore, the use of all-solid cell components eliminates the potential material corrosion and electrolyte management problems that are frequently experienced in high-temperature molten carbonate fuel cells. During the past several decades, a tremendous of research activities on SOFCs from both academic and industrial fields on the purpose of trying to develop them into practical devices have been envisioned.

The key part of SOFCs systems are the stacks, which are constructed from single cells, interconnectors, sealant and other auxiliary cell components. As the most important part of SOFCs, the single cell is typically composed of a porous cermet anode and a porous oxide cathode, with a dense electrolyte sandwiching them. To provide sufficient mechanical strength, the anode, the cathode or the electrolyte should be served as the support, and the corresponding cells are called anode-supported, cathode-supported and electrolyte-supported SOFCs, respectively. For the SOFCs with oxygen-ionic conducting electrolyte such as yttira-stabilized zirconia (YSZ), the cathode functions as the catalyst for electrocatalytic reduction of molecular oxygen to oxygen ions by providing electrons that are transferred from the anode, while the anode catalyzes the oxidation of fuel with oxygen ions that diffused from the cathode through the electrolyte with the simultaneous release of electrons which are transported through an external circuit to the cathode. Such electrons can be utilized to do work when an external load is connected. The electrolyte is a pure ionic conductor, typically an oxygen-ionic conductor. The interconnector is another important component of SOFCs that separates the anode and cathode into two gas chambers to avoid the direct mixing of fuel and oxidant. In addition, it functions as a current collector. As the interconnector is exposed to both the extremely reducing atmosphere of the anode and the highly oxidant atmosphere of the cathode, it must have sufficiently high chemical stability over the wide oxygen potential window. To ensure efficient current collection, it also needs to maintain high conductivity in both reducing and oxidizing atmospheres. Doped LaCrO₃ perovskites are the state-of-the-art material for interconnectors in high-temperature SOFCs [3]. Recently, stainless steels have been proposed as alternative interconnectors in intermediate-temperature (IT) SOFCs due to the economical and processing benefits [4]. However, the use of commercial ferritic stainless steels presents serious problems leading to degradation of the fuel cell stack, particularly on the cathode side. These problems include oxidation ferritic stainless steels by air and volatilization of Cr resulting in cathode Cr poisoning. To overcome these issues, the surface of metallic interconenctor exposing to the cathode atmosphere is usually deposited with a thin, dense, and conductive coatings by advanced techniques. Minh [5] has written an excellent comprehensive overview on the whole spectra of the SOFCs technologies achieved up to 1993. Tremendous progress has been made since then in SOFCs technologies (e.g., materials development, single cell design, cell fabrication, and cell stacking) and in the fundamental understanding of the mechanism and kinetics of electrode reactions in SOFCs. A significant number of review articles have appeared since then [3], [4], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], Table 1 lists most of the recent related review articles.

Although there are huge advantages associated with SOFCs, their widespread application has not yet been realized up to now. This is mainly because the cost associated with fuel cell fabrication, materials and maintenance is still too high to be competitive with current matured electric power generation technology from fire power plants. To accelerate the commercialization of SOFCs technology, the cost must be dramatically decreased. Economic competitiveness for this technology has been suggested at a system cost with an upper limit of $800/kW and a stretch limit of $400/kW [19]. The cost challenge requires the fuel cell stack to constitute no more than one fifth of the system cost (i.e., the stack cost should be approximately between $80 and $165 per kilowatt) [19]. On the other hand, the operational stability of SOFCs should also be substantially increased. For stationary power generation, a degradation rate of less than 1% per 1000 h is preferred. This rate suggests that the fuel cell should have a lifetime of at least 50,000 h [47]. To satisfy above requirements, the fuel cell materials, material synthesis and cell fabrication techniques, cell configurations, and stack designs should all be optimized.

State-of-the-art SOFCs are composed of YSZ electrolyte and typically operate at ∼1000 °C [5]. Such a high temperature is beneficial for improving the electrode reaction kinetics and reducing the electrolyte ohmic drop. Unfortunately, it also sets strict requirements on the cell materials. Both the sintering and the phase reaction between the cell components are obviously accelerated with increasing operation temperature. Operation at the intermediate range of 500–800 °C offers the choice of low-cost metallic materials, such as stainless steels for SOFCs interconnection and construction materials, which would make both the stack and balance-of-plant cheaper. On the other hand, the drop in operation temperature also alleviates the solid-phase reaction between the cell components, effectively reducing the maintenance cost and prolonging the lifetime of the fuel cell system. However, as the operation temperature decreases, a significant increase in electrochemical resistance in both the electrolyte and electrodes is frequently observed. As a result, the conventional SOFCs that use a thick YSZ electrolyte and a strontium-doped lanthanum manganese (LSM) cathode would deliver a poor power output, making them less attractive for practical use.

As the electrolyte ohmic resistance is in reverse proportion to the membrane thickness, a thin-film electrolyte configuration is proposed to reduce the ohmic drop at reduced operation temperatures, in this case, anode or cathode should be served as the substrate. However, the manufacturing cost of the thin-film electrolyte could dominate the fabrication cost of SOFCs. Will et al. [40] have reviewed different thin-film deposition methods for oxides, especially for stabilized zirconia. Recently, Beckel et al. [15] reviewed thin film (with thickness ⩽1 μm) deposition techniques related to SOFCs, including current research on nano-crystalline thin-film electrolytes and thin film-based model electrodes. Nowadays, cost-effective and easy scale-up techniques (e.g., tape casting and screen printing), which have been widely applied in industrial processes for the fabrication of solar cells and sensors [48], [49], have also been adopted in the fabrication of thin-film electrolytes for SOFCs. Thicknesses down to 10 μm have been achieved with great success [19].

For the fabrication of single cell with a thin-film electrolyte, typically the anode-supported thin-film electrolyte dual layer cells are first prepared by tape-casting, screen-printing or another effective technique. These green cells are then sintered at high temperature to allow for densification of the electrolyte layer. The other electrode is deposited onto the sintered electrolyte surface using advanced techniques. This is followed by firing at elevated temperature to make an adequate connection between the electrode particles and to obtain good adhesion of the electrode layer to the electrolyte surface. This minimizes the contact resistance and ensures the sufficient mechanical strength of the electrode layer. Sometimes triple-layer cells are sintered in a single step to simplify the preparation process and, thus, reduce the fabrication cost. As the anode and electrolyte are co-sintered, overheating could result in severe sintering of the electrode. However, sufficient porosity is needed for the free transportation of oxygen, fuels and reaction products within the electrode channels. Otherwise, a serious concentration polarization could occur, especially at high polarization currents. Thus, the sintering behavior of the anode and the electrolyte layers should be carefully tailored. Electrolyte sintering is closely related to the properties of starting powder, which in turn is determined by the powder synthetic technique. By applying nano-sized powders, the sintering temperature for the densification of the electrolyte could be several hundred degrees lower than for the conventional coarse powder [50]. On the other hand, the electrolyte conductivity is also closely related to the starting powder. It has been reported that nano-sized grains of YSZ electrolyte show an enhancement of one-to-two orders of magnitude in the specific grain boundary conductivity after the grain size of the solid electrolyte is reduced to the nano-size range [51]. The synthetic technique could also have a significant impact on the phase purity of the powder. Some impurity phases like SiO₂, could result in a decrease in the ionic conductivity of the Sm₂O₃-doped CeO₂ (SDC) electrolyte by several orders of magnitude [52].

With a reduction in operation temperature, another large contribution to the significant increase in cell resistance is the electrode polarization resistance, especially from the cathode side [53], [54], [55]. The state-of-the-art SOFCs cathode is the LSM cathode. Jiang [9] has reviewed and updated the development, understanding, and achievements of the LSM-based cathodes for SOFCs. However, LSM is not active enough below 800 °C. Its negligible ionic conductivity limits the electrochemical reaction strictly to the triple phase boundary (TPB) region [56], [57]. To extend the active zone for the oxygen reduction reaction (ORR), mixed ionic and electronic conductors (MIECs) are proposed as the cathode materials. Their mixed conductivity can effectively extend the active oxygen reduction sites from the typical TPB region to the entire exposed cathode surface, thus greatly reducing the cathode polarization at low operation temperatures. Great efforts have been made in the development of novel cathode materials. Up to now, several promising materials have been developed, including Sm_xSr_1−xCoO_3−δ (SSC), La_xSr_1−xCo_yFe_1−yO_3−δ (LSCF) and Ba_xSr_1−xCo_yFe_1−yO_3−δ (BSCF) [58], [59], [60], [61], [62], [63], [64]. However, such materials are usually quite reactive with the YSZ electrolyte, causing the formation of an insulating layer, which greatly increases the interfacial polarization resistance. Consequently, a buffer layer has been applied to avoid the direct contact of the electrolyte with the electrode [65], [66]. Recently, it has been demonstrated that the electrode can be directly fabricated onto the electrolyte surface without a phase reaction by adopting an advanced synthesis technique [67], [68]. On the other hand, the electrochemical performance also strongly depends on the microstructure of the cathodes, which in turn is strongly related to the properties of the starting powders [69], [70].

The development of new electrode and electrolyte materials, as well as advanced electrode fabrication techniques, is critical in reducing operation temperature and subsequently lessening the cost of SOFCs. Futhermore, optimization of synthetic techniques for starting powders is also important for the reduction of fabrication costs. In other ceramic material applications, such as lithium-ion batteries [71], ferroelectric ceramics [72] and superconducting materials [73], the significant effect that the properties of starting powders on their performance has been recognized and well reviewed. In open literature, many review papers about the low-temeprature cathode materials and advanced fabrication techniques for thin-film electrolytes or new electrolyte materials are available [21], [39], [40], [44], [45], however, a systematic review concerning the effect of synthetic methods on the performance of SOFCs is still lack. This paper provides a timely review on the synthesis of the electrodes, electrolyte and interconnect starting powders for use in SOFCs. The effect of different processes on their electrochemical performance is evaluated and the methods towards the optimization of synthesis processes are proposed.